The secondary battery is widely used not only as a power source for a portable device such as a mobile phone, a digital camera, or a laptop computer but also as a power source for a vehicle or a household, and a lightweight lithium-ion secondary battery having a high energy density among others is now an energy storage device indispensable to our life style.
The secondary batteries can be largely classified as a wound type and a laminated type. The battery assembly of the wound type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet are wound a plurality of times with a separator interposed between positive electrode sheet and negative electrode sheet which are stacked. The battery assembly of the laminated type secondary battery has a structure in which positive electrode sheets and negative electrode sheets are alternately stacked together repeatedly with a separator interposed between each positive and negative electrode sheet. The positive electrode sheet and the negative electrode sheet include coated portions in which active material (including combined agent that include binder, conductive material or the like) are applied to collectors, and uncoated portions in which any active material are applied to collectors so that the uncoated portions can connect to an electrode terminal.
In both the wound type secondary battery and the laminated type secondary battery, the battery assembly is sealed in an exterior container so that one end of a positive electrode terminal can be electrically connected to the uncoated portion of the positive electrode sheet, the other end can be drawn out of the exterior container (exterior case), one end of a negative electrode terminal can be electrically connected to the uncoated portion of the negative electrode sheet, and the other end can be drawn out of the exterior container. An electrolytic solution is sealed together with the battery assembly in the exterior container. The trend is for an increase in secondary battery capacity year by year which may lead to higher heat generation that results in greater safety issues of short-circuiting. Thus, battery safety measures are becoming more important.
As an example of the safety measures, in order to prevent short-circuiting between the positive electrode and the negative electrode, there is known a technology for forming an insulating member in a boundary portion between the coated portion and the uncoated portion (Patent Document 1).